Electric power steering control device

ABSTRACT

A gain target value is set to a usual value when battery voltage is within a usual range, and to a value below the usual value when the battery voltage is outside the usual range. When the battery voltage is recovered from outside the usual range to the usual range, it is determined whether or not a ratio obtained by dividing a time-integrated value of a gain in a period from when the battery voltage goes outside the usual range to when being recovered to the usual range by a time-integrated value of the gain when the gain in the period is the usual value is larger than a prescribed threshold. When determined to be larger than the prescribed threshold, the gain is set to immediately increase up to the gain target value, otherwise the gain is set to gradually increase up thereto.

TECHNICAL FIELD

The present invention relates to an Electric power steering controldevice.

BACKGROUND ART

Conventionally, electric power steering control devices (for example,see PTL 1) have been proposed to reduce an assist force when anelectrical abnormality is detected in hardware, such as a motor or atorque sensor. In the electric power steering control device describedin PTL 1, assist force is immediately increased if abnormality durationis less than a prescribed time when there is no electrical abnormalitydetection after detecting an electrical abnormality. This preventsgiving a steering feeling that the steering wheel suddenly feels heavy.Additionally, if the abnormality duration is equal to or more than theprescribed time, the assist force is gradually increased, which preventsgiving the steering feeling that the steering wheel suddenly becomeslight.

CITATION LIST Patent Literature

PTL 1: JP Pat. No. 4581535

SUMMARY OF INVENTION Technical Problem

However, such an electric power steering control device requires furtherimprovement in steering feeling.

The present invention has focused on the problem as above, and it is anobject of the present invention to provide an electric power steeringcontrol device capable of improving steering feeling.

Solution to Problem

To achieve the above object, according to an aspect of the presentinvention, there is provided an electric power steering control deviceincluding: (a) a motor configured to receive electrical power from abattery and output an assist force for assisting steering with asteering wheel; (b) a battery voltage sensor configured to detect avoltage of the battery; (c) a target value setting unit configured toset a gain target value, which is a target value of a gain used tocontrol the assist force output by the motor, on a basis of the voltagedetected by the battery voltage sensor; (d) a gain setting unitconfigured to set the gain on a basis of the gain target value set bythe target value setting unit; (e) a torque sensor configured to detecta steering torque applied by the steering wheel; and (f) a control unitconfigured to control the assist force output by the motor on a basis ofthe gain set by the gain setting unit and the steering torque detectedby the torque sensor, (g) wherein the target value setting unit sets thegain target value to a predetermined usual value when the voltagedetected by the battery voltage sensor is within a predetermined rangeof usual voltage, and sets the gain target value to a value below theusual value when the voltage is outside the range of the usual voltage;and (h) wherein when the voltage detected by the battery voltage sensoris recovered from outside the range of the usual voltage to within therange of the usual voltage, the gain setting unit determines whether ornot a ratio obtained by dividing a time-integrated value of the gain ina period from when the voltage goes outside the range of the usualvoltage to when the voltage is recovered to within the range of theusual voltage by a time-integrated value of the gain when the gain inthe period is the usual value is larger than a predetermined prescribedthreshold, the gain setting unit setting the gain to immediatelyincrease up to the gain target value when the ratio is determined to belarger than the prescribed threshold, and setting the gain to graduallyincrease up to the gain target value when the ratio is determined to beequal to or less than the prescribed threshold.

Advantageous Effects of Invention

According to the one aspect of the present invention, for example, whenthe time-integrated value of the gain from when the voltage of thebattery goes outside the range of the usual voltage to when the voltagethereof is recovered to within the range of the usual voltage is large,the gain is immediately increased up to the gain target value, which canthus reduce the time during which the steering wheel feels heavy.

Additionally, when the time-integrated value of the gain is small, thegain is gradually increased up to the gain target value, which can thusprevent the steering wheel from becoming light suddenly. Accordingly,there can be provided an electric power steering control device capableof improving steering feeling.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the structure of an electric powersteering control device according to a present embodiment;

FIG. 2 is a diagram illustrating the inner structure of an ECU;

FIG. 3 is a diagram illustrating the inner structure of an MCU;

FIG. 4 is a flowchart illustrating gain setting processing;

FIG. 5 is a diagram illustrating the state of gain recovery;

FIG. 6 is a diagram illustrating the state of gain recovery;

FIGS. 7A and 7B are diagrams illustrating how the voltage of a batteryfluctuates, in which FIG. 7A is a diagram illustrating the operatingstate of a starter motor, and FIG. 7B is a diagram illustrating thefluctuating state of the voltage of the battery;

FIGS. 8A to 8C are diagrams illustrating operation of the electric powersteering control device, in which FIG. 8A is a diagram illustrating thevoltage of the battery, FIG. 8B is a diagram illustrating gain targetvalue, and FIG. 8C is a diagram illustrating gain;

FIGS. 9A to 9C are diagrams illustrating operation of the electric powersteering control device, in which FIG. 9A is a diagram illustrating thevoltage of the battery, FIG. 9B is a diagram illustrating gain targetvalue, and FIG. 9C is a diagram illustrating gain;

FIGS. 10A to 10C are diagrams illustrating operation of the electricpower steering control device, in which FIG. 10A is a diagramillustrating the voltage of the battery, FIG. 10B is a diagramillustrating gain target value, and FIG. 10C is a diagram illustratinggain;

FIGS. 11A to 11C are diagrams illustrating operation of the electricpower steering control device, in which FIG. 11A is a diagramillustrating the voltage of the battery, FIG. 11B is a diagramillustrating gain target value, and FIG. 11C is a diagram illustratinggain;

FIGS. 12A to 12C are diagrams illustrating operation of the electricpower steering control device, in which FIG. 12A is a diagramillustrating the voltage of the battery, FIG. 12B is a diagramillustrating gain target value, and FIG. 12C is a diagram illustratinggain;

FIG. 13 is a diagram illustrating the inner structure of an MCUaccording to a modification;

FIG. 14 is a flowchart illustrating processing for setting a firstrecovery rate; and

FIG. 15 is a diagram illustrating the structure of an electric powersteering control device according to a modification.

DESCRIPTION OF EMBODIMENTS

The present inventors have found the following problems in conventionalelectric power steering control devices. In the convention electricpower steering devices, when a starter motor connected to engine isstarted up, a large current momentarily flows through the starter motor,and battery voltage drops, as illustrated in FIGS. 7A and 7B. Then, thebattery voltage drop reduces assist force, which can give a steeringfeeling that the steering wheel suddenly feels heavy.

In addition, when a tire rides up on a curb and receives a steeringreaction force, a motor regenerative current is generated, which maymomentarily increase battery current. Even in this case, assist force isreduced for circuit protection, whereby the steering wheel suddenlyfeels heavy, and when the state continues for a certain period of time,there may be given a discomfort in operational feeling.

Hereinafter, an example of an electric power steering control deviceaccording to an embodiment of the present invention will be describedwith reference to FIGS. 1 to 15. It should be noted that the presentinvention is not limited to the following example. Additionally, effectsdescribed in the present specification are merely examples and notintended to be limiting, and there may be other effects than those.

(Entire Structure of Electric Power Steering Control Device)

FIG. 1 is a diagram illustrating the entire structure of the electricpower steering control device according to the embodiment of the presentinvention. An electric power steering control device 1 of FIG. 1 isapplied to a column type electric power steering (EPS) that provides anassist force on a steering shaft 3 side.

As illustrated in FIG. 1, the electric power steering control device 1of the present embodiment includes a steering wheel 2, a steering shaft3, a pinion input shaft 4, a torque sensor 5, a speed reductionmechanism 6, a rack and pinion 7, rods 8L and 8R, a motor 9, a steeringangle sensor 10, a vehicle speed sensor 11, a battery 12, and anelectric control unit (ECU) 13.

One end side of the steering shaft 3 is connected to the steering wheel2. An other end side of the steering shaft 3 is connected to an inputside of the torque sensor 5. An output side of the torque sensor 5 isconnected to one end side of the pinion input shaft 4. The torque sensor5 is composed of one torsion bar and two resolvers each attached to eachend of the torsion bar to sandwich the torsion bar, in which one endside of the torsion bar is an input end and an other end side thereof isan output end. The two resolvers detect an amount of distortion or thelike of the torsion bar that occurs between the input and output ends,whereby a steering torque T applied by the steering wheel 2 is detected.The detected steering torque T is output to the ECU 13.

The speed reduction mechanism 6 is connected on the way of the pinioninput shaft 4. The speed reduction mechanism 6 transmits an assist forceoutput from the motor 9 to another end side of the pinion input shaft 4.Additionally, on the other end side of the pinion input shaft 4 isformed a pinion gear that can engage in a rack groove of a rack shaftforming the rack and pinion 7. The rack and pinion 7 converts rotationalmotion of the pinion input shaft 4 to linear motion of the rack shaft.In addition, the rods 8L and 8R are connected to both ends of the rackshaft. End portions of the rods 8L and 8R are connected to steeredwheels 14L and 14R via a knuckle or the like. As a result, when thepinion input shaft 4 rotates, actual steering angles of the steeredwheels 14L and 14R change via the rack and pinion 7, the rods 8L and 8R,and the like. In other words, it is possible to steer the steered wheels14L and 14R according to rotation of the pinion input shaft 4.

The steering angle sensor 10 detects a steering angle δ of the steeringwheel 2. The vehicle speed sensor 11 detects a vehicle speed V. Thedetected steering angle δ and vehicle speed V are output to the ECU 13.

The battery 12 supplies electrical power to various electricalcomponents of a vehicle mounted with the electric power steering controldevice 1, such as the motor 9, the ECU 13, a starter motor, a car airconditioner, a car navigation, and an audio system. Additionally, thebattery 12 is charged with electrical power generated by an alternator.

As illustrated in FIG. 2, the ECU 13 includes a torque detection circuit15, a steering angle detection circuit 16, a vehicle speed detectioncircuit 17, a motor current sensor 18, a battery voltage sensor 19, acharge pump circuit 20, a charge pump voltage sensor 21, a micro controlunit (MCU) 22, a field effect transistor (FET) driver 24, and a motordrive FET 25.

The steering torque T is input to the torque detection circuit 15 fromthe torque sensor 5. Additionally, the steering angle δ is input to thesteering angle detection circuit 16 from the steering angle sensor 10.The vehicle speed V is input to the vehicle speed detection circuit 17from the vehicle speed sensor 11. Each of the input steering torque T,steering angle δ, and vehicle speed V is output to the MCU 22. Inaddition, the motor current sensor 18 detects current values iU, iV, andiW of a current flowing through the motor 9. FIG. 2 illustrates anexample using a three-phase motor including a U-phase coil, a V-phasecoil, and a W-phase coil as the motor 9. The current value iU is acurrent flowing through the U-phase coil, the current value iV is thecurrent value of a current flowing through the V-phase coil, and thecurrent value iW is the current value of a current flowing through theW-phase coil. The detected current values iU, iV, and iW of the motor 9are output to the MCU 22. Furthermore, the battery voltage sensor 19detects a voltage of the battery 12. The detected voltage of the battery12 is output to the MCU 22.

The charge pump circuit 20 boosts the voltage of the battery 12. Theboosted voltage is applied to the FET driver 24. The charge pump voltagesensor 21 detects the voltage boosted by the charge pump circuit 20. Thedetected voltage is output to the MCU 22.

The MCU 22 includes a memory 26. The memory 26 stores various kinds ofprograms executable by the MCU 22. In addition, the memory 26sequentially stores various kinds of data when the various kinds ofprograms are executed. Examples of the data sequentially stored at theexecution of the programs include a gain G, a gain target value G*, atime-integrated value A, a time-integrated value B, a ratio R, and thelike, which will be described later.

For example, a random access memory (RAM) can be used as the memory 26.

In addition, when an ignition key is switched from an OFF-state to anON-state, the MCU 22 reads an assist force control program from thevarious kinds of programs stored in the memory 26, and executes theprogram. Then, through the assist force control program, a currentcommand value calculation unit 27, a current command value correctionunit 28, a three-phase conversion unit 37, and a pulse width modulation(PWM) drive unit 23 are realized by software, as illustrated in FIG. 3.

The current command value calculation unit 27 calculates a currentcommand value for controlling the assist force output by the motor 9using an assist map on the basis of the steering torque T output fromthe torque detection circuit 15 and the vehicle speed V output from thevehicle speed detection circuit 17. The assist map to be used is, forexample, a map for outputting a current command value according to theinput steering torque T and vehicle speed V when they are input. Thecalculated current command value is output to the current command valuecorrection unit 28.

The current command value correction unit 28 multiplies the currentcommand value output from the current command value calculation unit 27by the gain G set by the gain setting unit 33 that will be describedlater to obtain a corrected current command value. The gain G is set toa numerical value below “1.0” when the voltage of the battery 12 isoutside a predetermined range of usual voltage, and when the voltage ofthe battery 12 goes from outside the range of the usual voltage towithin the range thereof, the gain G is continuously changed from thenumerical value below “1.0” to “1.0”, and then maintained at “1.0”. Anexample of the usual voltage that can be employed is a normal voltage ofthe battery 12 at which the motor 9 can appropriately output an assistforce. Thus, when the voltage of the battery 12 is a normal voltage atwhich the motor 9 can appropriately output an assist force, the gain Gis maintained at “1.0”, and the current command value output from thecurrent command value calculation unit 27 becomes a corrected currentcommand value as it is. On the other hand, when the voltage of thebattery 12 is an abnormal voltage at which the motor 9 cannotappropriately output an assist force, the current command value outputfrom the current command value calculation unit 27 is reduced andbecomes a corrected current command value. The calculated correctedcurrent command value is output to the three-phase conversion unit 37.

The three-phase conversion unit 37 converts the corrected currentcommand value output from the current command value correction unit 28to a current command value of the U-phase coil of the motor 9, a currentcommand value of the V-phase coil thereof, and a current command valueof the W-phase coil thereof. Each of the converted current commandvalues is output to the PWM drive unit 23.

The PWM drive unit 23 calculates PWM signals for allowing the motor 9 tooutput an assist force according to magnitudes of the converted currentcommand values on the basis of the converted current command valuesoutput from the three-phase conversion unit 37. In other words, as theconverted current command values are larger, the PWM drive unit 23calculates PWM signals for allowing the motor 9 to output a largerassist force. As a method for calculating PWM signals, for example,there can be employed a method in which a PI control value is calculatedon the basis of differences (iU*−iU), (iV*−iV), and (iW*−iW) between theconverted current command values iU*, iV*, and iW* and the currentvalues iU, iV, and iW of the motor 9 detected by the motor currentsensor 18, and PWM calculation is performed on the basis of thecalculated PI control value to calculate a PWM signal corresponding tothe U-phase coil of the motor 9, a PWM signal corresponding to theV-phase coil thereof, and a PWM signal corresponding to the W-phase coilthereof. The calculated PWM signals are output to the FET driver 24.

The FET driver 24 uses electrical power from voltage applied by thecharge pump circuit 20 to drive the motor drive FET 25 according to thePWM signals output by the PWM drive unit 23.

The motor drive FET 25, when driven by the FET driver 24, useselectrical power supplied from the battery 12 to supply drive current tothe motor 9. The current command value calculation unit 27, the currentcommand value correction unit 28, the three-phase conversion unit 37,the PWM drive unit 23, the FET driver 24, and the motor drive FET 25 areincluded in a control unit 29 configured to control an assist forceoutput by the motor 9 on the basis of the gain G output from the gainsetting unit 33 and the steering torque T output from the torquedetection circuit 15.

In the electric power steering control device 1 structured as above, thetorque sensor 5 detects the steering torque T applied by the steeringwheel 2. Then, the MCU 22 of the ECU 13 calculates a current commandvalue according to the steering torque T and the like, furthermore, thePWM drive unit 23 outputs PWM signals on the basis of the currentcommand value, and the FET driver 24 drives the motor drive FET 25 onthe basis of the PWM signals. As a result, in the electric powersteering control device 1, the motor drive FET 25 supplies drive currentto the motor 9, whereby the motor 9 generates an assist force, which canassist the driver in steering with the steering wheel 2.

Additionally, the MCU 22 reads a gain setting program, simultaneouslywith the assist force control program, from the memory 26, and executesthe programs. Then, an initialization unit 30, a voltage acquisitionunit 31, a target value setting unit 32, and a gain setting unit 33 arerealized by the gain setting program. The initialization unit 30, thevoltage acquisition unit 31, the target value setting unit 32, and thegain setting unit 33 execute gain setting processing.

(Gain Setting Processing)

Next, a description will be given of the gain setting processingexecuted by the initialization unit 30, the voltage acquisition unit 31,the target value setting unit 32, and the gain setting unit 33.

As illustrated in FIG. 4, first, at step S101, the initialization unit30 executes initialization processing. Specifically, the initializationunit 30 performs processing for setting a prescribed initial value in aprescribed region of the memory 26. As a result, each of the gain G andthe gain target value G* stored in the prescribed region of the memory26 is set to “1.0”. Additionally, each of the time-integrated values Aand B is set to “0”.

Next, proceeding to step S102, the voltage acquisition unit 31 acquiresthe voltage of the battery 12 output from the battery voltage sensor 19.The voltage of the battery 12 is acquired every few hundred μs to fewms. Note that, for example, singular voltage data or a moving averagevalue or the like based on plural (three or more) voltage data may beused as the voltage of the battery 12.

Next, proceeding to step S103, the target value setting unit 32determines within which of the usual voltage, a first low voltageregion, a second low voltage region, a first high voltage region, and asecond high voltage region the voltage of the battery 12 acquired atstep S102 lies. Additionally, a magnitude relationship between therespective regions is as follows: second high voltage region>first highvoltage region>usual voltage>first low voltage region>second low voltageregion. Then, when the target value setting unit 32 determines that thevoltage of the battery 12 is within the range of the usual voltage,processing proceeds to step S104. On the other hand, when the voltage ofthe battery 12 is determined to be within the first low voltage regionor the first high voltage region, processing proceeds to step S114.Furthermore, on the other hand, when the voltage of the battery 12 isdetermined to be within the second low voltage region or the second highvoltage region, processing proceeds to step S118.

At step S104, the gain setting unit 33 determines whether or not thevoltage of the battery 12 acquired at step S102 has been recovered fromoutside the range of the usual voltage to within the range thereof. As amethod for determining whether or not there has been the recovery, forexample, there can be used a method in which when the voltage of thebattery 12 has been determined to be within the first low voltage regionor the first high voltage region at step S103 executed at previous cycleof the immediately preceding step S103, the voltage of the battery 12acquired at step S102 is determined to have been recovered from outsidethe range of the usual voltage to within the range thereof. Then, whenthe gain setting unit 33 determines that the voltage of the battery 12has been recovered (Yes), processing proceeds to step S105. On the otherhand, when the voltage thereof is determined not to have been recovered(No), processing proceeds to step S110.

At step S105, the time-integrated value B stored in the prescribedregion of the memory 26 is divided by the time-integrated value A tocalculate the ratio R.

At step S106, the gain setting unit 33 determines whether or not theratio R obtained at step S105 is larger than a predetermined prescribedthreshold Rth (for example, “0.5”). Then, when the ratio R is determinedto be larger than the prescribed threshold Rth of “0.5” (Yes),processing proceeds to step S107. On the other hand, when the ratio R isequal to or less than the prescribed threshold Rth of “0.5” (No),processing proceeds to step S108.

At step S107, the gain setting unit 33 sets a fluctuation rate dG/dt tobe used at step S112 or S113 to a relatively fast first recovery ratedG₁/dt, and then, processing proceeds to step S109.

At step S108, the gain setting unit 33 sets the fluctuation rate dG/dtto be used at step S112 or S113 to a relatively slow second recoveryrate dG₂/dt, and then, processing proceeds to step S109. The secondrecovery rate dG₂/dt used is a rate slower than the first recovery ratedG₁/dt.

At step S109, the gain setting unit 33 clears each of thetime-integrated values A and B stored in the prescribed region of thememory 26 and sets them to “0”.

Next, proceeding to step S110, the target value setting unit 32 sets thegain target value G* to a predetermined usual value (for example,“1.0”). Note that the usual value to be used can be any numerical valuethat is larger than the prescribed threshold Rth of “0.5”, and anumerical value other than “1.0” may be used.

Next, proceeding to step S111, the gain setting unit 33 determineswhether or not to perform characteristic control at recovery to set thefirst recovery rate dG₁/dt or the second recovery rate dG₂/dt inconsideration of at least any of the steering torque T, the steeringangle δ, the steering angular velocity dδ/dt, the vehicle speed V, andthe like. Then, when the characteristic control at recovery isdetermined not to be performed (No), processing proceeds to step S112.On the other hand, when the characteristic control at recovery isdetermined to be performed (Yes), processing proceeds to step S113.

At step S112, the gain setting unit 33 sets the gain G on the basis ofthe fluctuation rate dG/dt set at the step S107 or S108 and the gaintarget value G* of “1.0” set at step S110. Specifically, the gainsetting unit 33 adds a multiplication result obtained by multiplying thefluctuation rate dG/dt set at the step S107 or S108 by an update timet_(o) to the gain G stored in the memory 26 so that the gain Gapproaches the gain target value G* of “1.0”, and overwrites the gain Gstored in the memory 26 with a result of the addition. The update timet_(o) used is an elapsed time between a previous overwrite of the gain Gstored in the memory 26 and the present time.

In addition, simultaneously, the control unit 29 controls the assistforce output by the motor 9 on the basis of the gain G stored in thememory 26 and the steering torque T output from the torque sensor 5.Specifically, the current command value correction unit 28 multipliesthe gain G stored in the memory 26 by a current command value calculatedby the current command value calculation unit 27 to obtain a correctedcurrent command value.

Then, the three-phase conversion unit 37 converts the corrected currentcommand value to current command values of the U-phase coil, V-phasecoil, and W-phase coil of the motor 9. Next, the PWM drive unit 23outputs PWM signals of the U-phase coil, V-phase coil, and W-phase coilof the motor 9 on the basis of the converted current command values. TheFET driver 24 drives the motor drive FET 25 according to the output PWMsignals, and the motor drive FET 25 supplies drive current to the motor9. Then, returning to step S102, changes of stepsS102→S103→S104→S110→S111→S112 are repeated to repeat the setting of thegain G and the supply of the drive current to the motor 9, whereby thegain G is immediately or gradually increased up to the gain target valueG*, as a result of which the assist force output by the motor 9 isimmediately or gradually increased.

On the other hand, at step S113, the gain setting unit 33 sets the gainG on the basis of the fluctuation rate dG/dt set at step S107 or S108and the gain target value G* of “1.0” set at step S110. Specifically, asillustrated in FIGS. 5 and 6, an adding rate dG_(a)/dt such that thegain G increases in a first order straight lineG=a·t+b is calculated.Here, a and b are fixed values. In FIG. 6, T₁ represents a time whenrecovery of the gain G has started (when the voltage has returned towithin the range of the usual voltage), T₂ represents a time when therecovery thereof is complete, a₁ represents the gain G at the time whenthe recovery thereof has started, and a₂ represents the gain G (=1.0) atthe time when the recovery thereof is complete. When these are appliedto the above equation: G=a·t+b, the results are as follows:a=(a₂−a₁)/(T₂−T₁) and b=a₁.

Then, the gain setting unit 33 adds a multiplication result obtained bymultiplying an addition value of the calculated adding rate dG_(a)/dtand the fluctuation rate dG/dt set at step S107 or S108 by the updatetime t_(o) to the gain G stored in the memory 26 so that the gain Gapproaches the gain target value G* of “1.0”, and overwrites the gain Gstored in the memory 26 with a result of the addition.

Note that while the above description has described an example using theadding rate dG_(a)/dt that linearly increases the gain G, otherstructures may be employed. For example, first, it is determined whetheror not at least any of the steering torque T, the steering angle δ, thesteering angular velocity dδ/dt, and the vehicle speed V is larger thana prescribed threshold. For example, it is determined whether any one ormore of four conditions: (1) an absolute value of the steering torque Tis larger than a first threshold; (2) an absolute value of the steeringangle δ is larger than a second threshold; (3) an absolute value of thesteering angular velocity dδ/dt is larger than a third threshold; and(4) the vehicle speed V is larger than a fourth threshold are satisfied.When determined to be satisfied, at least any of the steering torque T,the steering angle δ, the steering angular velocity dδ/dt, and thevehicle speed V is determined to be larger than the prescribedthreshold. Then, when determined to be larger than the prescribedthreshold, there may be used the adding rate dG_(a)/dt that increasesthe gain Gin a slow curve that is a downwardly convex quadratic curveG=a·t²+b.

On the other hand, when it is determined to be smaller than theprescribed threshold, there may be used the adding rate dG_(a)/dt thatincreases the gain G in an early rising curve that is an upwardly convexcurve G=a·t^(1/2)+b. Note that “2” included in the functions of “t²” and“t^(1/2)” in the above quadratic curves may be regarded as a variable,and higher order functions, such as “t³” and “t^(1/3)” illustrated inFIG. 5, may be used.

In addition, simultaneously, the control unit 29 controls the assistforce output by the motor 9 on the basis of the gain G stored in thememory 26 and the steering torque T output from the torque sensor 5.Specifically, the current command value correction unit 28 multipliesthe gain G stored in the memory 26 by a current command value calculatedby the current command value calculation unit 27 to obtain a correctedcurrent command value.

Then, the three-phase conversion unit 37 converts the corrected currentcommand value to current command values of the U-phase coil, V-phasecoil, and W-phase coil of the motor 9. Next, the PWM drive unit 23outputs PWM signals of the U-phase coil, V-phase coil, and W-phase coilof the motor 9 on the basis of the converted current command values. TheFET driver 24 drives the motor drive FET 25 according to the output PWMsignals, and the motor drive FET 25 supplies drive current to the motor9. Then, returning to step S102, changes of stepsS102→S103→S104→S110→S111→S113 are repeated to repeat the setting of thegain G and the supply of the drive current to the motor 9, whereby thegain G is immediately or gradually increased up to the gain target valueG*, as a result of which the assist force output by the motor 9 isimmediately or gradually increased.

On the other hand, at step S114, the target value setting unit 32 setsthe gain target value G* to a set value (for example, “0.5”). The setvalue can be any numerical value between the usual value of “1.0” and“0”, and a numerical value other than “0.5” may be used.

Then, proceeding to step S115, the gain setting unit 33 sets the gain Gon the basis of the gain target value G* set at step S114. Specifically,a multiplication result obtained by multiplying a prescribed ratedG_(o1)/dt by the update time t_(o) is added to or subtracted from thegain G stored in the memory 26 so that the gain G approaches the gaintarget value G*, and the gain G stored in the memory 26 is overwrittenwith a result of the calculation. The update time t_(o) used is anelapsed time between a previous overwrite of the gain G stored in thememory 26 and the present time.

As a result, the control unit 29 controls the assist force output by themotor 9 on the basis of the gain G stored in the memory 26 and thesteering torque T output from the torque sensor 5. Specifically, thecurrent command value correction unit 28 multiplies the gain G stored inthe memory 26 by a current command value calculated by the currentcommand value calculation unit 27 to obtain a corrected current commandvalue.

Next, the three-phase conversion unit 37 converts the corrected currentcommand value to current command values of the U-phase coil, V-phasecoil, and W-phase coil of the motor 9. Then, the PWM drive unit 23outputs PWM signals of the U-phase coil, V-phase coil, and W-phase coilof the motor 9 on the basis of the converted current command values, theFET driver 24 drives the motor drive FET 25 according to the output PWMsignals, and the motor drive FET 25 supplies drive current to the motor9. The setting of the gain target value G* at step S114 and the settingof the gain G and the supply of the drive current to the motor 9 at stepS115 as described above are repeated many times to gradually reduce orincrease the gain G down or up to the gain target value G*, therebygradually reducing or increasing the assist force output by the motor 9.

Next, proceeding to step S116, the gain setting unit 33 adds amultiplication result obtained by multiplying the gain G set at stepS115 by the update time t_(o) to the time-integrated value B of the gainG stored in the memory 26. This calculates the time-integrated value Bof the gain G in a period (hereinafter referred to also as “abnormalperiod”) from when the voltage of the battery 12 goes outside the rangeof the usual voltage to when the voltage is recovered to within therange of the usual voltage.

Then, proceeding to step S117, the gain setting unit 33 adds amultiplication result obtained by multiplying the usual value of “1.0”by the update time t_(o) to the time-integrated value A of the gainstored in the memory 26, and then, processing returns to step S102. Thiscalculates the time-integrated value A of the gain G when the gain G inthe abnormal period is the usual value of “1.0”.

On the other hand, at step S118, the target value setting unit 32 setsthe gain target value G* to “0”.

Next, proceeding to step S119, the gain setting unit 33 sets the gain Gon the basis of the gain target value G* set at step S118. Specifically,as in step S115, a multiplication result obtained by multiplying aprescribed rate dG_(o2)/dt by the update time t_(o) is added to orsubtracted from the gain G stored in the memory 26 so that the gain Gapproaches the gain target value G*, and the gain G stored in the memory26 is overwritten with a result of the calculation. The update timet_(o) used is an elapsed time between a previous overwrite of the gain Gstored in the memory 26 and the present time.

As a result, the control unit 29 controls the assist force output by themotor 9 on the basis of the gain G stored in the memory 26 and thesteering torque T output from the torque sensor 5. Specifically, thecurrent command value correction unit 28 multiplies a current commandvalue calculated by the current command value calculation unit 27 by thegain G stored in the memory 26 to obtain a corrected current commandvalue.

Next, the three-phase conversion unit 37 converts the corrected currentcommand value to current command values of the U-phase coil, V-phasecoil, and W-phase coil of the motor 9. Then, the PWM drive unit 23outputs PWM signals based on the current command values of the U-phasecoil, V-phase coil, and W-phase coil of the motor 9 on the basis of theconverted current command values, the FET driver 24 drives the motordrive FET 25 according to the output PWM signals, and the motor driveFET 25 supplies drive current to the motor 9. The setting of the gaintarget value G* at step S118 and the setting of the gain G and thesupply of the drive current to the motor 9 at step S119 as describedabove are repeated many times to gradually reduce or increase the gain Gdown or up to the gain target value G*, thereby gradually reducing orincreasing the assist force output by the motor 9.

Next, proceeding to step S120, the gain setting unit 33 adds amultiplication result obtained by multiplying the gain G set at stepS119 by the update time t_(o) to the time-integrated value B of the gainG stored in the memory 26. This calculates the time-integrated value Bof the gain in the period (abnormal period) from when the voltage of thebattery 12 goes outside the range of the usual voltage to when thevoltage thereof is recovered to within the range of the usual voltage.

Then, proceeding to step S121, the gain setting unit 33 adds amultiplication result obtained by multiplying the usual value of “1.0”by the update time t_(o) to the time-integrated value A of the gainstored in the memory 26, and then, processing returns to step S102. Thiscalculates the time-integrated value A of the gain G when the gain G inthe abnormal period is the usual value of “1.0”.

Note that while the gain setting processing of the present embodimenthas been described by the example where the gain target value G* is setto “0.5” when the voltage of the battery 12 is within the first lowvoltage region or the first high voltage region, and to “0” when thevoltage thereof is within the second low voltage region or the secondhigh voltage region, the numerical values “0.5” and “0” are merelyexamples and not intended to be limiting, and other numerical values maybe used.

(Operation and Others)

Next, operation of the electric power steering control device 1according to the embodiment of the present invention will be describedwith reference to the drawings.

First, as illustrated in FIGS. 7A and 7B, assume that the starter motorconnected to the engine has been started, and a large current hasmomentarily flowed through the starter motor, whereby the voltage of thebattery 12 has started to drop, as illustrated at time to of FIG. 8A.Next, as illustrated at time t₁ of FIG. 8A, assume that the voltage ofthe battery 12 has changed from within the range of the usual voltage tothe first low voltage region. Then, as illustrated at time t₁ of FIG.8B, the ECU 13 maintains the gain target value G* at a set value (forexample, “0.5”) between a usual value (for example, “1.0”) and “0”, andreduces the gain G to approach the gain target value G* maintained atthe set value of “0.5”, as illustrated at time t₁ of FIG. 8C. Thiscontrols the motor 9 so that the assist force output by the motor 9 isslowly reduced down to 50% of an assist force output when the voltage ofthe battery 12 is within the range of the usual voltage (hereinafterreferred to also as “usual assist force”).

Next, assume that after the voltage of the battery 12 has stoppeddropping and the voltage value of the battery 12 has become constant, asillustrated at time t₂ of FIG. 8A, recovery (increase) of the voltage ofthe battery 12 has started, as illustrated at time t₃ of FIG. 8A, andthe voltage of the battery 12 has been recovered from the first lowvoltage region to within the range of the usual voltage, as illustratedat time t₄ of FIG. 8A. Then, the ECU 13 stops maintaining the gaintarget value G* at the set value of “0.5” and maintains the gain targetvalue G* at the usual value of “1.0”, as illustrated at time t₄ of FIG.8B, and increases the gain G to approach the gain target value G* set atthe usual value of “1.0”, as illustrated at time t₄ of FIG. 8C.

In this case, assume that the ratio R obtained by dividing thetime-integrated value B of the gain G in a period (hereinafter referredto also as “abnormal period C”) from when the voltage of the battery 12goes outside the range of the usual voltage to when the voltage thereofis recovered to within the range of the usual voltage by thetime-integrated value A of the gain G when the gain G in the abnormalperiod C is the usual value of “1.0” is larger than the prescribedthreshold Rth of “0.5” (for example, R=0.8). Then, the relatively fastfirst recovery rate dG₁/dt is employed as the fluctuation rate dG/dt ofthe gain G. As a result, as illustrated from time t₄ to time t₅ of FIG.8C, the gain G immediately increases up to the gain target value G* of“1.0”, and the motor 9 is controlled so that the assist force output bythe motor 9 immediately increases up to the usual assist force.

On the other hand, as in time to and time t₁ of FIG. 8A, assume that thevoltage of the battery 12 has started to drop, as illustrated at time t₆of FIG. 9A, and the voltage of the battery 12 has changed from withinthe range of the usual voltage to the first low voltage region, asillustrated at time t₇ of FIG. 9A. Then, the ECU 13 maintains the gaintarget value G* at the set value of “0.5”, as illustrated at time t₇ ofFIG. 9B, and reduces the gain G to approach the gain target value G*maintained at the set value of “0.5”, as illustrated at time t₇ of FIG.9C. Next, as illustrated at time t₈ of FIG. 9A, when the voltage of thebattery 12 changes from the first low voltage region to the second lowvoltage region, the ECU 13 stops maintaining the gain target value G* atthe set value of “0.5” and maintains the gain target value G* at “0”, asillustrated at time t₈ of FIG. 9B, and reduces the gain G to approachthe gain target value G* maintained at “0”, as illustrated at time t₈ ofFIG. 9C. This controls the motor 9 so that the assist force is reduceddown to “0”, as illustrated at time t₈ of FIG. 9C.

Then, when recovery of the voltage of the battery 12 starts, asillustrated at time t₉ of FIG. 9A, and the voltage of the battery 12changes from the second low voltage region to the first low voltageregion, as illustrated at time do of FIG. 9A, the ECU 13 stopsmaintaining the gain target value G* at “0” and maintains the gaintarget value G* at the set value of “0.5”, as illustrated at time t₁₀ ofFIG. 9B, and increases the gain G to approach the gain target value G*maintained at the set value of “0.5”, as illustrated at time t₁₀ of FIG.9C. Next, assume that the voltage of the battery 12 has been recoveredfrom the first low voltage region to within the range of the usualvoltage, as illustrated at time t₁₁ of FIG. 9A. Then, the ECU 13 stopsmaintaining the gain target value G* at the set value of “0.5” andmaintains the gain target value G* at the usual value of “1.0”, asillustrated at time t₁₁ of FIG. 9B, and increases the gain G to approachthe gain target value G* maintained at the usual value of “1.0”, asillustrated at time t₁₁ of FIG. 9C.

In this case, assume that the ratio R obtained by dividing thetime-integrated value B of the gain G in the abnormal period C by thetime-integrated value A of the gain G when the gain Gin the abnormalperiod C is the usual value of “1.0” is smaller than the prescribedthreshold Rth of “0.5” (for example, R=0.4). Then, the relatively slowsecond recovery rate dG₂/dt (<first recovery rate dG₁/dt) is employed asthe fluctuation rate dG/dt of the gain G. As a result, as illustratedfrom time t₁₁ to time t₁₂ of FIG. 9C, the gain G gradually increases upto the gain target value G* of “1.0”, and the motor 9 is controlled sothat the assist force output by the motor 9 gradually increases up tothe usual assist force.

Here, for example, as illustrated in FIGS. 10A, 10B, and 10C, assumethat the period (abnormal period C) from when the voltage of the battery12 goes outside the range of the usual voltage to when the voltagethereof is recovered to within the range of the usual voltage is short,and the ratio R is larger than the prescribed threshold Rth of “0.5”(for example, R=0.8). Then, the relatively fast first recovery ratedG₁/dt is employed as the fluctuation rate dG/dt of the gain G. As aresult, as illustrated from time t₁₁ to time t₁₂ of FIG. 10C, the gain Gimmediately increases up to the gain target value G* of “1.0”, and themotor 9 is controlled so that the assist force output by the motor 9immediately increases up to the usual assist force.

Thus, according to FIGS. 8A to 8C and 9A to 9C, the ratio R is largerthan the prescribed threshold Rth when the abnormal period C is long,whereas the ratio R is smaller than the prescribed threshold Rth whenthe abnormal period C is short, so that at first glance, it seems that amagnitude relationship between the ratio R and the prescribed thresholdRth is determined solely by the length of the abnormal period C.However, in the electric power steering control device 1 according tothe embodiment of the present invention, even when the abnormal period Cis short (4 ms), the ratio R is larger than the prescribed thresholdRth, as illustrated in FIGS. 10A to 10C. Accordingly, the structure isclearly different from one in which the magnitude relationship betweenthe ratio R and the prescribed threshold Rth is determined solely by thelength of the abnormal period C.

Additionally, on the other hand, assume that the tire of the steeredwheel 14L or 14R has ridden up on a curb and has received a steeringreaction force, thereby generating a regenerative current in the motor9, which has started to increase the voltage of the battery 12, asillustrated at time t₁₃ of FIG. 11A. Then, when the voltage of thebattery 12 changes from within the range of the usual voltage to thefirst high voltage region, as illustrated at time t₁₄ of FIG. 11A, theECU 13 maintains the gain target value G* at the set value of “0.5”, asillustrated at time t₁₄ of FIG. 11B, and reduces the gain G to approachthe gain target value G* maintained at the set value of “0.5”, asillustrated at time t₁₄ of FIG. 11C. This controls the motor 9 so thatthe assist force output by the motor 9 is slowly reduced down to 50% ofthe usual assist force.

Next, assume that after the voltage of the battery 12 has stoppedincreasing and the voltage value of the battery 12 has become constant,as illustrated at time t₁₅ of FIG. 11A, recovery (drop) of the voltageof the battery 12 has started, as illustrated at time t₁₆ of FIG. 11A,and the voltage of the battery 12 has been recovered from the first highvoltage region to within the range of the usual voltage, as illustratedat time t₁₇ of FIG. 11A. Then, the ECU 13 stops maintaining the gaintarget value G* at the set value of “0.5” and maintains the gain targetvalue G* at the usual value of “1.0”, as illustrated at time t₁₇ of FIG.11B, and increases the gain G to approach the gain target value G*maintained at the usual value of “1.0”, as illustrated at time t₁₇ ofFIG. 11C. In this case, assume that the ratio R obtained by dividing thetime-integrated value B of the gain G in the abnormal period C by thetime-integrated value A of the gain G when the gain G in the abnormalperiod C is the usual value of “1.0” is larger than the prescribedthreshold Rth of “0.5” (for example, R=0.8). Then, the relatively fastfirst recovery rate dG₁/dt is employed as the fluctuation rate dG/dt ofthe gain G. As a result, as illustrated from time t₁₇ to time t₁₈ ofFIG. 11C, the gain G immediately increases up to the gain target valueG* of “1.0”, and the motor 9 is controlled so that the assist forceoutput by the motor 9 immediately increases up to the usual assistforce.

Additionally, on the other hand, as in time t₁₃ and time t₁₄ of FIG.11A, assume that the voltage of the battery 12 has started to increase,as illustrated at time t₁₉ of FIG. 12A, and the voltage of the battery12 has changed from within the range of the usual voltage to the firsthigh voltage region, as illustrated at time t₂₀ of FIG. 12A. Then, theECU 13 maintains the gain target value G* at the set value of “0.5”, asillustrated at time t₂₀ of FIG. 12B, and reduces the gain G to approachthe gain target value G* maintained at the set value of “0.5”, asillustrated at time t₂₀ of FIG. 12C.

Next, when the voltage of the battery 12 changes from the first highvoltage region to the second high voltage region, as illustrated at timet₂₁ of FIG. 12A, the ECU 13 stops maintaining the gain target value G*at the set value of “0.5” and maintains the gain target value G* at “0”,as illustrated at time t₂₁ of FIG. 12B, and reduces the gain G toapproach the gain target value maintained at “0”, as illustrated at timet₂₁ of FIG. 12C. This controls the motor 9 so that the assist forceoutput by the motor 9 is reduced down to “0”, as illustrated at time t₂₁of FIG. 12C.

Next, when recovery of the voltage of the battery 12 starts, asillustrated at time t₂₂ of FIG. 12A, and the voltage of the battery 12changes from the second high voltage region to the first high voltageregion, as illustrated at time t₂₃ of FIG. 12A, the ECU 13 stopsmaintaining the gain target value G* at “0” and maintains the gaintarget value G* at the set value of “0.5”, as illustrated at time t₂₃ ofFIG. 12B, and increases the gain G to approach the gain target value G*maintained at the set value of “0.5”, as illustrated at time t₂₃ of FIG.12C. Next, assume that the voltage of the battery 12 has been recoveredfrom the first high voltage region to within the range of the usualvoltage, as illustrated at time t₂₄ of FIG. 12A. Then, the ECU 13 stopsmaintaining the gain target value G* at the set value of “0.5” andmaintains the gain target value G* at the usual value of “1.0”, asillustrated at time t₂₄ of FIG. 12B, and increases the gain G toapproach the gain target value G* maintained at the usual value of“1.0”, as illustrated at time t₂₄ of FIG. 12C. In this case, assume thatthe ratio R obtained by dividing the time-integrated value B of the gainG in the abnormal period C by the time-integrated value A of the gain Gwhen the gain G in the abnormal period C is the usual value of “1.0” issmaller than the prescribed threshold Rth of “0.5” (for example, R=0.4).Then, the relatively slow second recovery rate dG₂/dt (<first recoveryrate dG₁/dt) is employed as the fluctuation rate dG/dt of the gain G. Asa result, as illustrated from time t₂₄ to time t₂₅ of FIG. 12C, the gainG gradually increases up to the gain target value G* of “1.0”, and themotor 9 is controlled so that the assist force output by the motor 9gradually increases up to the usual assist force.

As described above, the electric power steering control device 1according to the embodiment of the present invention is configured toinclude the motor 9 configured to receive electrical power from thebattery 12 and output an assist force for assisting steering with thesteering wheel 2, the battery voltage sensor 19 configured to detect thevoltage of the battery 12, the target value setting unit 32 configuredto set the gain target value G*, which is the target value of the gain Gused to control the assist force output by the motor 9, on the basis ofthe voltage detected by the battery voltage sensor 19, the gain settingunit 33 configured to set the gain G on the basis of the gain targetvalue G* set by the target value setting unit 32, the torque sensor 5configured to detect the steering torque T applied by the steering wheel2, and the control unit 29 configured to control the assist force outputby the motor 9 on the basis of the gain G set by the gain setting unit33 and the steering torque T detected by the torque sensor 5.

Then, the target value setting unit 32 is configured to set the gaintarget value G* to the predetermined usual value of “1.0” when thevoltage detected by the battery voltage sensor 19 is within thepredetermined range of usual voltage, and set the gain target value G*to a value below the usual value of “1.0” when the voltage detected bythe battery voltage sensor 19 is outside the range of the usual voltage.Additionally, when the voltage detected by the battery voltage sensor 19is recovered from outside the range of the usual voltage to within therange thereof, the gain setting unit 33 determines whether or not theratio R obtained by dividing the time-integrated value B of the gain Gin the period (abnormal period C) from when the voltage of the battery12 goes outside the range of the usual voltage to when the voltagethereof is recovered to within the range of the usual voltage by thetime-integrated value A of the gain G when the gain G in the abnormalperiod C is the usual value of “1.0” is larger than the predeterminedprescribed threshold Rth of “0.5”. When the ratio R is determined to belarger than the prescribed threshold Rth of “0.5”, the gain setting unit33 sets the gain G to immediately increase up to the gain target valueG*. On the other hand, when the ratio R is determined to be equal to orless than the prescribed threshold Rth of “0.5”, the gain setting unit33 sets the gain G to gradually increase up to the gain target value G*.

Therefore, for example, as illustrated in FIGS. 8C, 10C, and 11C, whenthe time-integrated value B of the gain G from when the voltage of thebattery 12 goes outside the range of the usual voltage to when thevoltage thereof is recovered to within the range of the usual voltage islarge as compared to the time-integrated value A of the gain G when thegain G is the usual value of “1.0”, the gain G is immediately increasedup to the gain target value G*, which can thus reduce the time duringwhich the steering wheel 2 feels heavy. Additionally, as illustrated inFIGS. 9C and 12C, when the time-integrated value B of the gain G whenthe voltage of the battery 12 has been recovered to within the range ofthe usual voltage is small as compared to the time-integrated value A ofthe gain G when the gain G is the usual value of “1.0”, the gain G isgradually increased up to the gain target value G*, which can thusprevent the steering wheel 2 from becoming light suddenly. Accordingly,the electric power steering control device 1 capable of improvingsteering feeling can be provided.

Additionally, the electric power steering control device 1 according tothe embodiment of the present invention is configured to set the gaintarget value G* on the basis of the voltage of the battery 12, set thegain G on the basis of the gain target value G*, and control the assistforce on the basis of the gain G, so that deterioration of steeringfeeling of the driver can be suppressed. This is particularly effectivein case of battery voltage fluctuations during steering, and the like.

Incidentally, if the gain target value G* is not set to a value below“1.0” when the voltage of the battery 12 is outside the range of theusual voltage, the battery 12 may be deteriorated, failure of otherequipment may be induced, or the motor drive FET 25 may be thermallydamaged. Additionally, if the gain G is set to immediately increase upto the gain target value G* even when the ratio R is equal to or lessthan the prescribed threshold Rth, there may be felt a steeringdiscomfort as if the torque has been lost.

In contrast, in the electric power steering control device 1 accordingto the embodiment of the present invention, the gain target value G* isset to a value below “1.0” when the voltage of the battery 12 is outsidethe range of the usual voltage, which can thus prevent deterioration ofthe battery 12, induced failure of other equipment, and thermal damageto the motor drive FET 25. In addition, when the ratio R is equal to orless than the prescribed threshold Rth of “0.5”, the gain G is set togradually increase up to the gain target value G*, which can thusprevent the driver from feeling the steering discomfort as if the torquehas been lost.

(Modifications)

(1) The electric power steering control device 1 according to theembodiment of the present invention has been described by exemplifyingthe case where the increase rate (at least one of the first recoveryrate dG₁/dt and the second recovery rate dG₂/dt, which is hereinafterreferred to as “recovery rate”) of the gain G when the gain setting unit33 increases the gain G up to the gain target value G* is set to aconstant value. However, other structures may be employed. For example,the recovery rate may be set according to the value of the ratio R.Specifically, the larger the ratio R, the larger the recovery rate maybe set, whereas the smaller the ratio R, the smaller the recovery ratemay be set.

(2) Additionally, for example, the gain setting unit 33 may beconfigured to set the recovery rate on the basis of at least one or acombination of two or more of the ratio R, the steering torque T, thesteering angle δ, the steering angular velocity dδ/dt, and the vehiclespeed V. Specifically, for example, as illustrated in FIG. 13, the MCU22 may realize a steering angular velocity calculation unit 34 bysoftware, and the gain setting unit 33 may include a recovery ratesetting unit 35 and a recovery rate correction unit 36.

The steering angular velocity calculation unit 34 calculates thesteering angular velocity dδ/dt on the basis of the steering angle δoutput from the steering angle detection circuit 16. The calculatedsteering angular velocity dδ/dt is output to the gain setting unit 33.

The recovery rate setting unit 35 sets the recovery rate to a largervalue as the ratio R is larger, and to a smaller value as the ratio R issmaller. Additionally, the recovery rate correction unit 36 corrects therecovery rate set by the recovery rate setting unit 35 on the basis ofat least one or a combination of two or more of the steering torque T,the steering angle δ, the steering angular velocity dδ/dt, and thevehicle speed V. For example, when at least any of the steering torqueT, the steering angle δ, the steering angular velocity dδ/dt, and thevehicle speed V is smaller than a prescribed threshold and it isdifficult to feel a discomfort in steering feeling, the recovery rateset by the recovery rate setting unit 35 is increased. On the otherhand, when at least any of the steering torque T, the steering angle δ,the steering angular velocity dδ/dt, and the vehicle speed V is equal toor more than the prescribed threshold and it is easy for the driver tofeel a discomfort in steering feeling, the recovery rate set by therecovery rate setting unit 35 is reduced. This allows the recovery ratecorrection unit 36 to appropriately operate as a rate limiter, andenables the gain G to be quickly recovered to a usual state whileminimizing deterioration of steering feeling.

(3) In addition, another method for setting the recovery rate on thebasis of the steering torque T, the steering angle δ, the steeringangular velocity dδ/dt, the vehicle speed V, and the like may be amethod not using correction. Specifically, when setting the fluctuationrate dG/dt at step S107, the gain setting unit 33 executes processingfor setting the first recovery rate dG₁/dt. When the processing forsetting the first recovery rate dG₁/dt is executed, the gain settingunit 33 first determines, at step S201, as illustrated in FIG. 14,whether or not to perform characteristic control at recovery to set thefirst recovery rate dG₁/dt in consideration of the steering torque T andthe like. Then, when the characteristic control at recovery isdetermined not to be performed (No), processing proceeds to step S202.On the other hand, when the characteristic control at recovery isdetermined to be performed (Yes), processing proceeds to step S203.

At step S202, the gain setting unit 33 sets the first recovery ratedG₁/dt so that the gain G increases in a first order straight lineG=a·t+b, as illustrated in FIGS. 5 and 6. Here, a and b represent fixedvalues. In FIG. 6, T₁ represents a time when recovery of the gain G hasstarted (when the voltage has returned to within the range of the usualvoltage), T₂ represents a time when the recovery thereof is complete, a₁represents the gain G at the time when the recovery thereof has started,and a₂ represents the gain G (=1.0) at the time when the recoverythereof is complete. When these are applied to the above equation:G=a·t+b, the results are as follows: a=(a₂−a₁)/(T₂−T₁) and b=a₁.

On the other hand, at step S203, the gain setting unit 33 determineswhether at least any of the steering torque T, the steering angle δ, thesteering angular velocity dδ/dt, and the vehicle speed V is larger thana prescribed threshold. For example, it is determined whether any one ormore of four conditions: (1) an absolute value of the steering torque Tis larger than a first threshold; (2) an absolute value of the steeringangle δ is larger than a second threshold; (3) an absolute value of thesteering angular velocity dδ/dt is larger than a third threshold; and(4) the vehicle speed V is larger than a fourth threshold are satisfied.When determined to be satisfied, at least any of the steering torque T,the steering angle δ, the steering angular velocity dδ/dt, and thevehicle speed V is determined to be larger than the prescribedthreshold. Then, when determined to be larger than the prescribedthreshold, the first recovery rate dG₁/dt is set so that the gain Gincreases in a slow curve that is a downwardly convex quadratic curveG=a·t²+b, and then the setting processing is ended.

On the other hand, when determined to be smaller than the prescribedthreshold, the first recovery rate dG₁/dt is set so that the gain Gincreases in an early rising curve that is an upwardly convex curveG=a·t^(1/2)+b, and then the setting processing is ended. Note that “2”included in the functions of “t²” and “t^(1/2)” in the above quadraticcurves may be regarded as a variable, and higher order functions, suchas “t³” and “t^(1/3)”, may be used.

Furthermore, when setting the fluctuation rate dG/dt at step S108, thegain setting unit 33 executes processing for setting the second recoveryrate dG₂/dt, and sets the second recovery rate dG₂/dt in the same methodas the above-described method for setting the first recovery ratedG₁/dt. The second recovery rate dG₂/dt to be set is a rate slower thanthe first recovery rate dG₁/dt set at steps S202 and S203.

(4) Furthermore, while the present embodiment has described the examplewhere the electric power steering control device 1 according to thepresent invention is applied to the column type EPS in which a steeringassist force output by the motor 9 is applied to the steering shaft 3,other structures may be employed. For example, as illustrated in FIG.15, the electric power steering control device 1 may be applied to adownstream type EPS in which a steering assist force output by the motor9 is directly applied to a rack shaft.

REFERENCE SIGNS LIST

-   -   1: Electric power steering control device    -   2: Steering wheel    -   3: Steering shaft    -   4: Pinion input shaft    -   5: Torque sensor    -   6: Speed reduction mechanism    -   7: Rack and pinion    -   8L: Rod    -   8R: Rod    -   9: Motor    -   10: Steering angle sensor    -   11: Vehicle speed sensor    -   12: Battery    -   13: ECU    -   14L, 14R: Steered wheel    -   15: Torque detection circuit    -   16: Steering angle detection circuit    -   17: Vehicle speed detection circuit    -   18: Motor current sensor    -   19: Battery voltage sensor    -   20: Charge pump circuit    -   21: Charge pump voltage sensor    -   22: MCU    -   23: PWM drive unit    -   24: FET driver    -   25: Motor drive FET    -   26: Memory    -   27: Current command value calculation unit    -   28: Current command value correction unit    -   29: Control unit    -   30: Initialization unit    -   31: Voltage acquisition unit    -   32: Target value setting unit    -   33: Gain setting unit    -   34: Steering angular velocity calculation unit    -   35: Recovery rate setting unit    -   36: Recovery rate correction unit    -   37: Three-phase conversion unit

1. An electric power steering control device comprising: a motorconfigured to receive electrical power from a battery and output anassist force for assisting steering with a steering wheel; a batteryvoltage sensor configured to detect a voltage of the battery; a targetvalue setting unit configured to set a gain target value, which is atarget value of a gain used to control the assist force output by themotor, on a basis of the voltage detected by the battery voltage sensor;a gain setting unit configured to set the gain on a basis of the gaintarget value set by the target value setting unit; a torque sensorconfigured to detect a steering torque applied by the steering wheel;and a control unit configured to control the assist force output by themotor on a basis of the gain set by the gain setting unit and thesteering torque detected by the torque sensor, wherein the target valuesetting unit sets the gain target value to a predetermined usual valuewhen the voltage detected by the battery voltage sensor is within apredetermined range of usual voltage, and sets the gain target value toa value below the usual value when the voltage is outside the range ofthe usual voltage; and wherein when the voltage detected by the batteryvoltage sensor is recovered from outside the range of the usual voltageto within the range of the usual voltage, the gain setting unitdetermines whether or not a ratio obtained by dividing a time-integratedvalue of the gain in a period from when the voltage goes outside therange of the usual voltage to when the voltage is recovered to withinthe range of the usual voltage by a time-integrated value of the gainwhen the gain in the period is the usual value is larger than apredetermined prescribed threshold, the gain setting unit setting thegain to immediately increase up to the gain target value when the ratiois determined to be larger than the prescribed threshold, and settingthe gain to gradually increase up to the gain target value when theratio is determined to be equal to or less than the prescribedthreshold.
 2. The electric power steering control device according toclaim 1, wherein the gain setting unit sets a recovery rate, which is anincrease rate of the gain when increasing the gain up to the gain targetvalue, on a basis of at least one or a combination of two or more of theratio, the steering torque, a steering angle, a steering angularvelocity, and a vehicle speed.
 3. The electric power steering controldevice according to claim 2, wherein the gain setting unit includes arecovery rate setting unit configured to set the recovery rate to alarger value as the ratio is larger, and to a smaller value as the ratiois smaller.
 4. The electric power steering control device according toclaim 3, wherein the gain setting unit includes a recovery ratecorrection unit configured to correct the recovery rate set by therecovery rate setting unit on a basis of at least one or a combinationof two or more of the steering torque, the steering angle, the steeringangular velocity, and the vehicle speed.
 5. The electric power steeringcontrol device according to claim 1, wherein the control unit includes acurrent command value calculation unit configured to calculate a currentcommand value for controlling the assist force output by the motor on abasis of the steering torque detected by the torque sensor, a currentcommand value correction unit configured to multiply the current commandvalue calculated by the current command value calculation unit by thegain set by the gain setting unit to obtain a corrected current commandvalue, a three-phase conversion unit configured to convert the correctedcurrent command value obtained by the current command value correctionunit to current command values of a U-phase coil, a V-phase coil, and aW-phase coil of the motor, a PWM drive unit configured to generate pulsewidth modulation (PWM) signals on a basis of the converted currentcommand values converted by the three-phase conversion unit, and a FETdriver configured to, according to the PWM signals generated by the PWMdrive unit, drive a motor drive FET configured to supply drive currentto the motor.